The present invention relates to a fine red iron oxide pigment and a paint or a resin composition using the fine red iron oxide pigment. More particularly, the present invention relates to a fine red iron oxide pigment which is uniform in both major axial diameter and minor axial diameter thereof and exhibits an excellent transparency, and a paint or a resin composition using the fine red iron oxide pigment, which also exhibits an excellent transparency.
Hematite particles have been widely known as a red iron oxide pigment, because these particles can exhibit a red color and have been used in many applications such as coloring of paints, printing inks, plastics, films, cosmetics or the like.
Among the hematite particles, by using those particles having a particle size of not more than 0.1 .mu.m, the coating film obtained therefrom is transparent to visible light and, therefore, such particles are useful as a transparent red iron oxide pigment.
The red iron oxide pigment composed of fine hematite particles having a particle size of not more than 0.1 .mu.m (hereinafter referred to merely as "fine red iron oxide pigment") are deteriorated in dispersibility in vehicles or resin composition due to the reduction of particle size thereof. Therefore, a coating film or a resin composition using the fine red iron oxide pigment is unsatisfactory in transparency.
Namely, the fine red iron oxide pigment has a high surface energy and tends to be agglomerated due to the reduction of particle size thereof, so that it is difficult to disperse the pigment in vehicles or resin compositions. Therefore, a coating film obtained from such an insufficient dispersion cannot show a sufficient transparency since the fine pigment is agglomerated into coarse particles.
Consequently, it has been demanded to improve not only the dispersibility of the fine red iron oxide pigment in vehicles or resin compositions but also the transparency of the pigment itself.
Hitherto, as the method of enhancing the dispersibility of the fine red iron oxide pigment, there has been known a method of improving a particle size thereof. As such a method, there has been already proposed a method of producing fine hematite particles having a uniform particle size by first producing fine goethite particles having a uniform particle size in an aqueous solution and then heat-dehydrating the obtained fine goethite particles while maintaining the uniform particle size of the fine goethite particles (Japanese Patent Application Laid-Open (KOKAI) No. 49-34498(1974), Japanese Patent Publication (KOKOKU) No. 59-48768(1984), etc.).
Especially, in Japanese Patent Application Laid-Open (KOKAI) No. 49-34498(1974), there are described (1) a process for producing a fine iron oxide pigment comprising spindle-shape particles having a uniform particle size, a ratio of a major axial diameter to a minor axial diameter of not more than 5:1 and an average particle diameter of 5 to 20 nm, which process comprises a first step of adding caustic alkali such as sodium hydroxide to a solution of a ferrous salt such as ferrous sulfate to obtain a ferrous hydroxide colloid solution at a temperature of from room temperature to 40.degree. C.; a second step of reacting the obtained ferrous hydroxide with bicarbonate such as ammonium bicarbonate to obtain a ferrous carbonate colloid solution at a temperature of from room temperature to 40.degree. C.; and a third step of passing an oxygen-containing gas such as air through said ferrous carbonate colloid solution at a temperature of from room temperature to 40.degree. C. to transform said ferrous carbonate into ferric oxide hydroxide, and (2) a process for producing a fine iron oxide pigment, which process comprises (i) producing fine ferric oxide hydroxide comprising spindle-shape particles having a uniform particle size, a ratio of a major axial diameter to a minor axial diameter of not more than 5:1 and an average particle diameter of 5 to 20 nm by conducting a first step of adding caustic alkali such as sodium hydroxide to a solution of a ferrous salt such as ferrous sulfate to obtain a ferrous hydroxide colloid solution at a temperature of from room temperature to 40.degree. C.; a second step of reacting the obtained ferrous hydroxide with bicarbonate such as ammonium bicarbonate to obtain a ferrous carbonate colloid solution at a temperature of from room temperature to 40.degree. C.; and a third step of passing an oxygen-containing gas such as air through said ferrous carbonate colloid solution at a temperature of from room temperature to 40.degree. C. to transform said ferrous carbonate into ferric oxide hydroxide, (ii) after subjecting said ferric oxide hydroxide as a precipitate to washing with water, filtering-out and drying, dehydrating said precipitate at a temperature of 250 to 350.degree. C., thereby obtaining ferric oxide comprising spindle-shape particles which comprise primary particles having a particles diameter of about 5 nm, and have a uniform particle size and a ratio of a major axial diameter to a minor axial diameter of not more than 5:1.
In Japanese Patent Publication (KOKOKU) No. 59-48768(1984), there are described (1) a process for producing an iron oxide pigment having a uniform particle size, which process comprises (i) stirring a ferrous carbonate suspension produced by adding an aqueous solution of a ferrous salt such as ferrous sulfate to an aqueous solution of carbonate such as sodium carbonate, for 2 to 4 hours under a non-oxidation condition, to obtain a fine colloid solution, and (ii) while adjusting a pH value of said ferrous carbonate colloid solution to 7 to 10, passing an oxygen-containing gas such as air through the solution to produce ferric oxide hydroxide, and (2) a process for producing an iron oxide pigment having a uniform particle size, which process comprises (i) producing a fine colloid solution by stirring a ferrous carbonate suspension produced by adding an aqueous solution of a ferrous salt such as ferrous sulfate to an aqueous solution of carbonate such as sodium carbonate, for 2 to 4 hours under a non-oxidation condition; (ii) while adjusting a pH value of said ferrous carbonate colloid solution to 7 to 10, passing an oxygen-containing gas such as air through the solution to produce a precipitate of ferric oxide hydroxide; and (iii) after washing with water, filtering-out and drying, dehydrating said precipitate of ferric oxide hydroxide at a temperature of 250 to 500.degree. C.
At the present time, it has been most strongly demanded to provide a fine red iron oxide pigment having as uniform a particle size as possible in order to enhance the dispersibility thereof in vehicles or resin compositions. However, such fine red iron oxide pigment which is uniform in both major axial diameter and minor axial diameter thereof, cannot be obtained yet.
That is, in the above-mentioned known methods, as shown in Comparative Examples hereinafter, the fine goethite particles used as a starting material have failed to exhibit a sufficiently uniform particle size, especially a uniform minor axial diameter. Further, in the subsequent heat-dehydration process, the fine goethite particles tend to be sintered together due to the existence of ultrafine goethite particles mingled therein, so that the fine red iron oxide pigment obtained by the known methods has also failed to exhibit a uniform particle size, especially a uniform minor axial diameter.
On the other hand, in European Patent No. 919522 A, there have been proposed acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 150 m.sup.2 /g and an average major axis diameter of 0.004 to 0.295 .mu.m; and acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 150 m.sup.2 /g and an average major axis diameter of 0.004 to 0.295 .mu.m. and containing aluminum within the particle in an amount of 0.05 to 50% by weight (calculated as Al) based on the weight of the acicular hematite particles.
As a result of the present inventors' earnest studies, it has been found that in advance of a heat-dehydrating treatment of fine goethite particles at a temperature of 250 to 500.degree. C. to transform the fine goethite particles into fine hematite particles, by heat-treating the fine goethite particles at a temperature of 100 to 200.degree. C. to make the fine goethite particles absorb ultrafine goethite particles, the obtained hematite particles having a geometrical standard deviation of major axial diameter of not more than 1.5, a geometrical standard deviation of minor axial diameter of not more than 1.3 and an average major axial diameter of 0.005 to 0.1 .mu.m are useful as a fine red iron oxide pigment which is excellent in transparency, and a paint or a resin composition obtained by using the fine red iron oxide pigment can also show an excellent transparency. The present invention has been attained on the basis of the finding.